Electromagnetic wave generation is increased through highly efficient, high power consuming, and highly integrated electro-electronic devices. Electromagnetic waves cause malfunctions to other devices and systems or damage to the human body, so effective electromagnetic wave shield techniques are required in order to shield the electromagnetic waves.
EMI shielding effectiveness is represented by the following Equation 1.S.B.(shielding effectiveness)=R+A+B  [Equation 1]
In the above Equation 1, R represents surface reflection of an electromagnetic wave, A represents internal absorption of an electromagnetic wave, and B represents loss by multi-reflection.
The conventional method of shielding electromagnetic waves includes an applied painting and plating method using a metallic material.
Since the metallic material has high conductivity (R value, impedance is low) and a high electromagnetic wave shield rate through the surface reflection of electromagnetic waves, it is possible for even a thin metal to effectively shield electromagnetic waves.
However, the painting and plating techniques, particularly the plating process, include complicated steps such as removing oils, etching, neutralizing, activating, accelerating, metal depositing, activating, first plating, second plating, third plating, and so on. Accordingly these techniques have drawbacks such as high production costs and low productivity, particularly in view of recent demands for increased productivity.
In contrast, an electromagnetic wave shielding material using a polymer composite resin can be obtained by simply injecting a composite resin, so it is a very economical process with regard to production cost and productivity.
However, in the case of a composite material using the polymer composite resin, since the electrical conductivity is lower than that of a metallic material, it is important to improve the surface reflection and internal absorption among the factors shown in Equation 1. Accordingly, the resin composite material has the drawback of deteriorated or reduced electromagnetic wave shielding efficiency when it is too thin. In order to increase the electromagnetic wave shielding efficiency of a resin composite material, the surface impedance thereof is decreased (electrical conductivity is increased), the R value is increased, and internal electromagnetic wave scattering/absorption is further induced, so that the A value is increased to provide a highly effective electromagnetic wave shielding composite resin.
The following publications relate to shielding electromagnetic waves coming from all electronic devices such as radio frequency interference (RFI): an electromagnetic wave shielding device including a polymer substrate coated with metal on its surface (U.S. Patent Application Publication No. 2007-0199738); an electromagnetic wave shielding material including a non-conductive polymer, a conductive polymer, and an electrically conductive metal powder (U.S. Patent Application Publication No. 2007-0056769); a method of manufacturing an electrically conductive immersed fiber by coating a conductive fiber with a compatibilizer such as an organic wetting agent, and compositing the same in a resin (U.S. Patent Application Publication No. 2002-0108699); an electrically conductive thermoplastic elastomer including a conductive filler of nickel plated with silver in a styrene-ethylene-butadiene-styrene copolymer (SEBS) based matrix material which is a non-conductive resin (U.S. Pat. No. 6,638,448); an electrically conductive composition in which a carbonaceous conductive filler is immersed in a blend of two polymer resins having different polarities and the carbonaceous conductive filler is disposed on one having the higher polarity (U.S. Pat. No. 6,409,942); and a thermoplastic electromagnetic wave shielding sheet including a sheet material or polymer carrier that is capable of becoming porous during a thermoforming process and including a low-melting point metal conductive filler (U.S. Pat. No. 5,869,412).
However, these techniques provide resins with only electrical conductivity and thus do not satisfy the required electromagnetic wave shielding effects.